Am I more at risk of an STI if I pay for sex?

If you wear a condom while having vaginal sex and receiving a blow job, you are no more at risk than if you were not paying for sex or if you were to have casual sex.
A condom will protect you against hiv and the risk of other STIs is much smaller. Some STIs can be found around the edge of the condom. Also fingering one another and mutual masturbating can pose a risk.
100% safe sex is not possible: not with prostitutes and not with anybody else.